1. Field of the Invention
The invention generally relates to optical imaging of tissue and, more specifically, to second, third and fourth near-infrared spectral windows for deep optical imaging of tissue with less scattering.
2. Description of Prior Art
It is well known that light at wavelengths in the visible to near-infrared (NIR) range from 650 nm to 950 nm is a non-invasive optical tool to detect and image tissue abnormalities with Silicon based detectors. Optical mammography, for example, is an alternative NIR technique which utilizes NIR light to identify and image cancerous breast lesions. NIR light allows for greater depth penetration, minimal absorption and scattering into tissue than at shorter wavelengths in the visible region. The tissue becomes clearer in NIR region>900 nm. By choosing the appropriate wavelength of light and CCD detector, one can increase the penetration depth into tissue media and produce clearer optical images into NIR such as InGaAs and InSb detectors.
In 1929, Dr. Culter reported using white light and optical transillumination to image the breast [1]. He had hoped to replace to use of X-rays with longer wavelengths of light in the visible and NIR, however, due to lack of appropriate detectors and laser sources, he was unsuccessful. Since that time, better detectors, laser sources, and computer technologies have allowed others, such as B. Chance and R. R. Alfano, to use frequency modulation and time resolved imaging, respectively, to effectively image breast abnormalities [2, 3]. Nowadays, the NIR region with wavelengths from 650 nm to 950 nm, called the first therapeutic window, is conventionally used for most NIR tissue imaging studies and photodynamic therapy applications [4]. This NIR optical window shows less scattering and minimal absorption than in the visible range due to the inverse wavelength n>1 power dependence. At longer wavelengths, less scattering and blurring of optical images will occur. Just as the first optical window shows less scattering than in the visible range, it is expected that longer NIR wavelengths of light, above 950 nm, show less scattering and higher contrast images than the first optical window into >1000 nm.